Routing

Router functions

• Packet switching


• Packet filtering


• Internetwork communication


• Path selection

Routing protocols

Routing protocol functions:

• Learn routing information about IP subnets from other neighboring routers


• Advertise routing information about IP subnets to other neighboring routers


• If more than one possible route exists to reach one subnet, pick the best route based on a metric


• If the network topology changes—for example, a link fails—react by advertising that some routes have failed, and pick a new currently best route. This process is called convergence.)

Classfull and classless routing

This is the essence of classful routing: If one part of a major network is known, but the subnet toward which the packet is destined within that major network is unknown, the packet is dropped, even a default route is configured.

ip classless command will save you

The ip route command has two basic formats. The command can refer to a next-hop IP address. Alternately, for static routes that use point-to-point serial links, the command can list the outgoing interface instead of the next-hop IP address.

Static routing

Using static routes is appropriate with stub networks, for example the route from provider to client.

Distance vector loop prevention

• Route poisoning - when route fails, router sends this route with metric 16(infinity)


• Split horizon - In routing updates sent out interface X, do not include routing information about routes that refer to interface X as the outgoing interface. Poison reverse is used in Cisco instead of it.


• Triggered update - when a route fails, do not wait for the next periodic update. Instead, send an immediate triggered update listing the poisoned route.


• Poison reverse - when learning of a failed route, suspend split-horizon rules for that route, and advertise a poisoned route.


• Holddown - the holddown process tells a router to ignore new information about the failed route, for a time period called the holddown time

Default gateway

There are 3 methods to configure default gateway:

• ip default-gateway. It should only be used when ip routing is disabled on the Cisco router.


• ip default-network. When you configure ip default-network the router considers routes to that network for installation as the gateway of last resort on the router. For every network configured with ip default-network, if a router has a route to that network, that route is flagged as a candidate default route.


• ip route 0.0.0.0 0.0.0.0

Configuring default gateway

Troubleshooting

show ip protocols

Shows the parameters and current state of the active routing protocol process.